1. Field of the Invention
The present invention relates to a film thickness measurement apparatus of a thin film, and an electronic component fabrication apparatus including a film growth step. Particularly, the present invention relates to a thickness measurement apparatus measuring the thickness of various thin films formed on a substrate using the light interference method, and an electronic component fabrication apparatus.
2. Description of the Background Art
In the fabrication of electronic components such as of semiconductor devices and liquid crystal display devices, film growth technique such as plasma processing and sputtering is now widely used. By detecting various properties of a thin film formed by the film growth technique, various parameters for forming a thin film can be obtained. Also, various deficiencies during film growth can be promptly detected.
The thickness of a thin film greatly affects the pattern formation in thin films as well as to the properties such as conductivity and insulation thereof. Erroneous film thickness may cause disconnection and shorting in interconnection films and the like formed above the thin film. The film thickness is an important control factor that greatly influences the fabrication yield and reliability.
Conventionally, measurement of thickness of a thin film formed by a film growth apparatus is time consuming. Critical transportation and handling of the substrate are required so that the thin film is not damaged during thickness measurement. Accordingly, the measurement apparatus becomes so complex that it is difficult to introduce the measurement apparatus into an empty space of an existing line. In most cases, measurement was carried out off-line. The method of measuring the thickness of a thin film includes various methods such as the contact type method measuring a stepped portion in the thin film or using an ellipsometer.
Referring to FIG. 1, a film thickness measurement apparatus using an ellipsometer includes a polarizer 101 and an analyser 102. The light from a light source is polarized by polarizer 101 to be directed onto a thin film formation substrate 103 that is to be worked. Light reflected from substrate 103 is received by analyser 102 to have the polarized status of the reflected light detected by a detector. The detector compares the polarized status of incident light with that of the reflected light to obtain the optical constant (refractive index, attenuation coefficient) of the film thickness.
When a thin film layer 104 is formed on a substrate 106 covering a wiring pattern 105 of metal and the like as shown in FIG. 2, subtle unevenness is exhibited at that portion. Therefore, the film thickness could not be measured by the above-mentioned measurement method.
Thus, a particular region absent of wiring pattern 105 is selected and film growth effected thereon. Alternatively, a film is grown on a dummy substrate absent of wiring pattern, and the film thickness of an arbitrary point of that thin film is measured. Film growth conditions are determined to eliminate any deficiency in the thin film on the dummy substrate. Then, those film growth conditions are applied in the actual stage of production. Fabrication is effected on the assumption that a similar film is grown.
A method utilizing light interference is known as the measurement method relatively impervious to the unevenness on the substrate. This light interference method measures the thickness of a thin film by analyzing the spectrum of light reflected from or passing through the substrate.
An example of a light interference method will be described with the reference to FIG. 3. Light emitted from a light source is reflected from a substrate 103. The light reflected at the surface side of substrate 103 is a combination of light R1 reflected from the surface of thin film 104 and light R2 reflected from the surface of the main body 106 of substrate 103 excluding thin film 104.
The relationship between the wavelength and light intensity of the reflected light of FIG. 3 detected by a spectroscope will be described with reference to FIG. 4. The wavelength of the reflected light is plotted along the abscissa and light intensity is plotted along the ordinate in the graph. Light R1 and R2 interfere with each other, so that the light intensity becomes strong and weak with respect to the wavelength of the reflected light. This light interference occurs due to the difference in the light paths of light R1 and R2. Therefore, light interference depends on the thickness of thin film 104 and the angle of light radiation. The curve of the wavelength-light intensity in the graph is altered according to the thickness of thin film 104. Therefore, the thickness of film 104 can be obtained by analyzing the shape of the curve of the wavelength-light intensity in the graph depending upon the condition.
The peak valley method is known as the analyzing method of the curve of the wavelength-light intensity shown in FIG. 4. The wavelength corresponding to the peak in light intensity (points a and b in FIG. 4) in the wavelength-light intensity relationship curve is obtained. The film thickness is obtained from a relationship equation thereof.
As a film thickness measurement method using the wavelength-light intensity relationship curve, Japanese Patent Laying-Open No. 5-10726 discloses an invention of the film thickness measurement method utilizing transmitted light. In this measurement method, the curve of the relationship between the wavelength and light intensity of light transmitted through a transmittive substrate is obtained using a light source and a sensor. In addition to the peak valley method that obtains the wavelength corresponding to light intensity of the maximum level, a method is known of measuring the film thickness by altering the thickness and refractive index of the thin film so that the relationship curve approximates the wavelength-light intensity logic relationship curve as much as possible obtained from logic expressions that will be described afterwards. The following relationship is established where T is the transmittance of the thin film, n′ is the refractive index of the thin film, n′0 is the refractive index of air, n′1 is the refractive index of the transparent substrate, r0 is the amplitude reflectance when light enters the thin film from air, r1 is the amplitude reflectance when light enters the transparent substrate from the thin film, δ is the offset in phase when light travels through the thin film, δ0 is the offset in phase when light enters the thin film from air, and δ1 is the offset in phase when light enters the transparent substrate from the thin film.
                    T        =                                            4              ×                              n                                  ′                  2                                            ×                                                (                                                            n                      0                      ′                                        +                                          n                      1                      ′                                                        )                                2                                                                                      (                                                            n                      ′                                        +                                          n                      0                      ′                                                        )                                2                            ×                                                (                                                            n                      ′                                        +                                          n                      1                      ′                                                        )                                2                                              ×                      1                          1              +                                                r                  0                                ×                                  r                  1                                            +                              2                ×                                  r                  0                                ×                                  r                  1                                ×                                  cos                  ⁡                                      (                                          δ                      +                                              δ                        0                                            +                                              δ                        1                                                              )                                                                                                          (        1        )            
Since phase offset δ when light passes through the thin film depends upon thickness d the thin film and light wavelength λ, the relationship between wavelength λ and thin film transmittance T can be obtained from equation (1). Therefore, thin film thickness d and thin film refractive index n′ are altered, and values thereof corresponding to the case where the wavelength-light intensity logic relationship curve obtained from equation (1) is closest to the wavelength-light intensity measured relationship curve are taken as the measurement values.
In the film thickness measurement apparatus employing an ellipsometer of FIG. 1, the position relationship of substrate 103 with respect to polarizer 101 and analyser 102 must be fixed. The thickness of the thin film cannot be measured when there is vertical oscillation, inclination, shifting and the like in substrate 103. Particularly in the fabrication line of a liquid crystal display device using a thin film glass substrate of approximately 0.5–1.1 mm in thickness and of a large size at least several hundred mm square, significant warping(partial inclination), oscillation, and like occur in the substrate. In order to use the film thickness measurement apparatus in-line, a robust large stage must be installed to be impervious to any partial inclination and oscillation. It is also necessary to mount polarizer 101, analyser 102 and substrate 103 in precise position to set the optical axis. Thus, it is difficult to measure a plurality of sites on substrate 103 simultaneously. Also, the film thickness measurement apparatus cannot be introduced in a limited space in-line since the measurement system is of a large scale in many cases.
In the method where a film is grown on a dummy substrate that is absent of a wiring pattern and measuring the film thickness of an arbitrary point of that thin film, there is a problem that the substrate of the product is not always identical to that of the dummy substrate even if optimum growth conditions are obtained by the dummy substrate and employed for the actual stage of production.
Furthermore, the extra step of carrying out film growth on a dummy substrate is required. This means that the number of steps required in the film thickness measurement process is increased. It may become necessary to reduce the number of measurement sites of film thickness for one product. The possibility of not detecting improper film thickness or delay in detecting improper film thickness may occur to result in significant damage.
Microscopic patterns of wiring and other metal films are formed between the substrate and the thin film in various electronic components. The above-described peak-valley method which is a light interference method is employed as the film thickness measurement method to have such influences relatively reduced. However, the peak valley method is disadvantageous in that thin film thickness measurement cannot be implemented in logic if there is only one peak or valley in the measured wavelength range, as shown in FIG. 5. Even if there are two or more peaks or valleys of light intensity, the peak position of light intensity will be shifted when light absorption by the thin film occurs in the wavelength range near the peak or the valley. Therefore, proper measurement of the film thickness cannot be obtained.
The film thickness measurement method disclosed in Japanese Patent Laying-Open No. 5-10726 analyzes the waveform per se of the wavelength-light intensity curve. Therefore, thickness of the thin film can be measured even if there is only one peak or valley of the light intensity. However, this measurement method does not take into account the absorption coefficient of the thin film, as apparent from equation (1). In the case where there is light absorption by the thin film in the measuring wavelength range, the wavelength range of measurement must be shifted to a wavelength range absent of absorption by the thin film. This means that many light sources differing in wavelength range must be prepared in the case of measuring a plurality of thin films. A mechanism to switch the light source according to the type of the thin film to be measured is required. Accordingly, the thin film measurement apparatus is increased in size and cost.
In the case where a multilayer film is grown on a substrate, the logic curve can be obtained by application of equation (1) in principle. However, a long period of time is required to obtain a curve that matches the actually measured wavelength-light intensity curve by altering all the parameters since the number of parameters increases in proportion to the number of the thin films. In the logic expression obtaining the thickness of a multi layer film, the total sum of the error of the thickness of each thin film will correspond to the error of the film thickness of the multilayer film. Thus, there is a problem that the logic curve does not easily match the actual measured curve.
Film thickness measurement is time consuming because light radiation of a large wavelength range and analysis thereof are required. Since only a transmittive type substrate is expected as the subject, there is also the problem that thin film measurement is difficult or the measured accuracy degraded in the case where the substrate is not transparent or when there is a light blocking film such as wiring between the thin film and the substrate.
It is noted that the position relationship of the substrate, the light source and the sensor must be fixed. The light path may be oscillated when the substrate is shifted vertically, inclined or oscillated. It was difficult to properly measure the film thickness. Particularly in the fabrication line of a liquid crystal display device using a thin film glass substrate of approximately 0.5–1.1 mm in thickness and of a large size at least several hundred mm square, significant warping(partial inclination), oscillation, and like occur in the substrate. In order to use the film thickness measurement apparatus in-line, a robust large stage must be installed to be impervious to any partial inclination and oscillation. Thus, the film thickness measurement apparatus is increased in size. Furthermore, the substrate and the sensor must be positioned accurately to set the optical axis. Therefore, it is difficult to measure a plurality of sites on a substrate simultaneously. Also, the sensor cannot be introduced in a limited space of the existing line in many cases.
A possible consideration is that the film thickness of a plurality of sites is to be measured while moving the sensor. However, it is difficult to move the sensor at high speed while maintaining the optical axis at high accuracy. This means that the measurement is time consuming.
It was difficult to measure the film thickness in-line by the conventional film thickness measurement methods. An overall or local defect in the substrate could be detected in the stage of inspection after several processing steps have already been carried out. There is a time lag from the occurrence of a defect until the defect is detected. Thus, there was a problem that a large amount of defective products will be generated during the time lag.
In the conventional film thickness measurement method, the breakdown or time for maintenance of the film thickness measurement apparatus could not be predicted due to change in various conditions such as degradation of the light source of the film thickness measurement apparatus. This is one cause of degrading the operating efficiency of the film thickness measurement apparatus.
In the invention disclosed in Japanese Patent Laying-Open No. 5–10726, a light source and a sensor are installed at both sides of the substrate since the measurement method employs transmitted light. In the case where the distance between the substrate and the light source or sensor is too small when the apparatus is employed in-line, the substrate will be brought into contact with the light source or the sensor due to oscillation or position shifting of the substrate to result in breakage or damage of the substrate.
Furthermore, thin film measurement utilizing transmitted light has the disadvantage that measurement is difficult since light is not easily transmitted in the region of the substrate where a reflective film is formed at a constant area ratio.
Furthermore, when thin films of a multilayer are formed on the substrate, each thin film cannot be calculated accurately unless the absorption coefficient of respective thin films are taken into account. There is a complicated distribution of reflected light and refracted light when a patterned reflective film that is concave and convex at a constant area ratio on the substrate is present, or when light is reflected and refracted at respective interfaces of the multilayer films. There was a problem that the distribution of the transmitted light becomes complex, whereby measurement of the thickness of the thin film becomes difficult.